Hardwood Floors February/March 2026

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Division 3 versus Division 9 Floor Flatness Tolerances (position statement #6)

1. Continuous measurement at any gap under the straightedge. 2. Indefinite number of straightedge locations on the floor. 3. No minimum or maximum number of readings. 4. Measurements typically taken with the straightedge crossing construction joints, or column blockouts, and near penetrations. 5. Measurements made just prior to the installation of the floor, which can be between

NOTICE: A disparity exists between concrete floor flatness tolerances and subfloor flatness tolerances specified for wood flooring at the time of installation. The following disparity has been adopted as detailed in the American Society of Concrete Contractors (ASCC) as published in “Concrete International, a publication of the American Concrete Institute (ACI)”: A. Division 3 specifications for concrete floor flatness typically include floor flatness requirements. The specifications also require that floor tolerance measurements be taken in accordance with ASTM E1155,

”Standard Test Method for Determining Floor Flatness (FF) and Floor Levelness (FL) Numbers.”

4-18 months after concrete placement. C. These two tolerances are obviously not compatible, nor measured with the same specifications. And floor flatness changes with time (due to curling of the slab) which makes it impossible to predict flatness when flooring is ready to be installed. To further complicate the issue, concrete contractors seldom receive Division 9 specification requirements when bidding the job, nor are floor coverings normally selected at this time. Concrete contractors are

Thus, the F-number measurements for meeting Division 3 requirements incorporate the following: 1. Point elevations

measured at regular 12” (300 mm) intervals along each line. 2. Measurement lines distributed uniformly across the test section. 3. Minimum # of

responsible for meeting the requirements of Division 3. D. It is recommended by NWFA, American Society of Concrete Contractors (ASCC), National Tile Contractors Association (NTCA), The Flooring Contractors Association (FCICA), Tile Contractors Association of America (TCAA), International Masonry Institute (IMI), and

readings required for statistical approach. 4. Measurement lines not within 2’ (.6 m) of any slab boundary, construction joint, isolation joint, block-out, penetration, or other similar discontinuity.

International Union of Bricklayers and Allied

5. F latness measured within

Craftworkers (BAC), that the owner of the project provide a bid allowance, established by

72 hours of concrete placement. B. Division 9 specifications for concrete

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the architect/engineer and based on the flooring requirements, for any necessary grinding and patching to close the gap between Division 3 and Division 9 tolerances. Providing an allowance enables the owner to compare floor covering bids on an equal basis.

floors to receive a wood floor provide floor flatness requirements in terms of an allowable gap (1/8” in 6’ or 3/16” in 10’) under an unleveled straightedge. There is no ASTM procedure for this measurement. Straightedge measurements for Division 9 incorporate the following:

Source: NWFA Wood Flooring Installation Guidelines (revised 2025).

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